Powder coating of polyepoxide and polyisocyanate-amine reaction product

ABSTRACT

Low-cure powder coating compositions are disclosed. The compositions comprise a polyepoxide and a material having the structure 
                         
wherein R 1  is an organic radical having 6 to 25 carbon atoms; R 2  is an organic radical having 1 to 20 carbon atoms; R 3  and R 4  are independently alkyl or phenyl groups having 1 to 8 carbon atoms; Z is oxygen or nitrogen, and when Z is oxygen R 5  is absent and when Z is nitrogen R 5  is hydrogen or is
 
                         
and n is 1 to 4. The material can optionally be reacted with an acidic hydrogen-containing compound. The compositions are curable without the use of crosslinking agents or accelerators. Methods for coating a substrate using these compositions, and substrates coated thereby, are also disclosed, as are additional catalysts useful for the same purpose.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/930,846, filed Oct. 31, 2007 (now abandoned), which is a continuationof U.S. patent application Ser. No. 11/758,170, filed Jun. 5, 2007, nowU.S. Pat. No. 7,390,765, which is a division of U.S. patent applicationSer. No. 11/190,666, filed Jul. 27, 2005, now U.S. Pat. No. 7,244,801,which is a continuation of U.S. patent application Ser. No. 10/669,947,filed Sep. 24, 2003, now abandoned, which is a division of U.S. patentapplication Ser. No. 10/160,466, filed May 31, 2002, now U.S. Pat. No.6,737,163.

FIELD OF THE INVENTION

The present invention relates to powder coating compositions; moreparticularly, the present invention relates to low temperature curethermosetting powder coating compositions. The compositions consistentlyproduce coatings having desirable performance properties when cured, andthat are stable when uncured.

BACKGROUND OF THE INVENTION

Coating compositions have long been used to provide the surface ofarticles with certain desired physical characteristics, such as color,gloss and durability. Many coating compositions rely on a liquidcarrier, which evaporates after the composition is applied. In recentyears, powder coatings have become increasingly popular; because thesecoatings are inherently low in volatile organic content (VOCs), theiruse reduces air emissions during the application and curing processes ascompared with liquid coatings.

Powder coatings are typically cured by heating the coated substrate toan elevated temperature. These temperatures almost always exceed 125°C., and commonly reach about 190° C. to 205° C. During the curingprocess, the powder particles melt and spread, and the components of thepowder coating react. In addition to not emitting any VOCs into theenvironment during the application or curing processes, powder coatingsystems are extremely efficient since there is essentially no waste(i.e., application yield is approximately 100 percent). Because of therelatively high (i.e., greater than 125° C.) cure temperatures of mostpowder coatings, their use, for practical purposes, is often limited tosubstrates that can withstand such high temperatures or that can beheated to an appropriate temperature long enough for cure to occur.

Despite the desirability of low-cure powder compositions, two problemshave prevented their widespread production and use—their mechanicalstability and their chemical stability. Powders that use resins with aglass transition temperature (“Tg”) lower than 60° C. usually encounterpackage stability problems, especially if exposed to prolonged heatexposure, and become fused, sintered or clumpy within days. Similarly,prolonged heat exposure can destroy the chemical stability of a powderif it includes crosslinkers that react at temperatures below about 170°C.; if a crosslinker with a lower cure temperature is used, cure may beinitiated during storage even though the film has not been formed. Thepremature gelation that occurs in these powder formulations results incoatings having shortened gel times. It is not unusual for low-curepowders to lose >50 percent of their gel time as a result of thepremature gelation.

Problems encountered when a powder loses either mechanical or chemicalstability can be severe. Poor mechanical stability creates obvioushandling, application and appearance issues. Poor chemical stabilitycreates subtler yet just as problematic issues. For example, a powderthat has poor chemical stability will fluidize and apply like virginpowder, but because it has advanced in reactivity (i.e. undergone somepremature gelation), it demonstrates restricted flow or no flow at allduring cure. The result can be a coating having an “orange peel”appearance, a rough texture or gel bodies.

Ideally, a powder should not lose its handling properties under elevatedtemperature storage and the gel time should remain the same as that ofthe virgin material. To achieve this, powders are typically formulatedwith resins having a Tg greater than about 60° C. and/or crosslinkersthat react at temperatures of about 170° C. or greater. Such powders,however, are not low cure. Low-cure powders having lower Tg resins orlower temperature crosslinkers can require expensive storage underrefrigeration and air-conditioned application facilities to overcomeinherent lack of stability, or must be prepared using specialtechniques.

Thus, there is a need in the coatings art for low-cure powder coatingshaving a wide range of application, which also have an acceptable levelof durability when cured on the finished product and good stability atroom temperature.

SUMMARY OF THE INVENTION

The present invention is directed to powder coating compositionsgenerally comprising a tertiary aminourea compound, a tertiaryaminourethane compound, or mixtures thereof, and a film-formingpolyepoxide resin. It has been surprisingly discovered that polyepoxideresins, when used with the present tertiary aminourea and/oraminourethane compositions, cure to form a suitable coating without theaid of crosslinkers, accelerators, or other additives typically regardedin the art as being necessary to cure a polyepoxide resin. The curedcoatings that result from the present compositions have performanceproperties that are at least as good as powder coating compositionsprepared with the same polyepoxides and conventional curing agents, butlacking the tertiary aminourea or aminourethane compositions describedherein. Significantly, this desirable result is achieved by using curingtemperatures much lower than those used for conventional products.Accordingly, the present compositions are low-cure. “Low-cure” as usedherein refers to powder coating compositions that cure at a temperaturebetween about 80° C. and 125° C. However, the present invention is notlimited to this temperature range and also provides cured films attemperatures up to and even greater than 190° C.

As a result of being low-cure, the present compositions can be used onsubstrates that are not appropriately exposed to temperatures greaterthan about 125° C. Examples include, but are not limited to, plasticssuch as thermoset and thermoplastic compositions, wood, and pieces ofthick metal that cannot be heated above about 95° C. because of theirsize. Also suitable are articles of manufacture that include a varietyof substrates; for example, motors that contain both metal and rubbercomponents can be suitably coated using the present, low-cure powdercompositions.

The present compositions also overcome some of the difficulties thathave been observed with other powder coating compositions, particularlyother low-cure powders. For example, the present powder compositions arestorage stable, and reduce, if not eliminate, the problems with chemicaland mechanical stability seen with other low-cure powder compositions.The present compositions can be stored at room temperature, and they donot continue to catalyze the reaction of the polyepoxide molecules afterthe removal of heat. Moreover, the present powder compositions can beprepared using standard methods known in the art for preparing powdercoating compositions; no special processing or handling is needed. Thus,the present compositions provide a significant advance in the low-curepowder coatings art.

Methods for coating substrates using the present powder compositions,and substrates coated thereby, are also within the scope of the presentinvention. Various low-cure catalysts are also included in the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to a powder coating compositioncomprising: (a) a material having the structure of Formula I:

wherein R₁ is an organic radical having 6 to 25 carbon atoms, R₂ is anorganic radical having 1 to 20 carbon atoms; R₃ and R₄ are independentlyalkyl or phenyl groups having 1 to 8 carbon atoms; Z is oxygen ornitrogen and when Z is oxygen R₅ is absent and when Z is nitrogen R₅ ishydrogen or

and n is 1 to 4; and (b) a polyepoxide. It will be understood that whenZ is oxygen a tertiary aminourethane compound is represented and when Zis nitrogen, Formula I depicts a tertiary aminourea compound. It will befurther understood that when R₅ is

there will be two each of R₂, R₃ and R₄. Each R₂, each R₃ and each R₄can be the same or different as the other R₂, R₃ or R₄. For example, oneR₂ can have one carbon and the other have two carbons, and the like.

The material of Formula I can be an oligomer wherein R₁ is a monovalent,divalent, trivalent or tetravalent organic radical; divalent radicalsare particularly suitable. The R₁ radical can be aliphatic, such ashexamethylene, cycloaliphatic such as cyclohexylene, substitutedcycloaliphatic such as 1,1,3,3-tetramethylcyclohexylene, or aromaticsuch as phenylene. Substituted cycloaliphatics are particularlysuitable, especially 1,1,3,3-tetramethylcyclohexylene. Examples ofsuitable R₂ moieties include ethylene, n-propylene, and iso- andn-butylene. In a particularly suitable composition, Z is nitrogen, R₁ is1,1,3,3-tetramethylcyclohexylene, R₂ is propylene, R₃ and R₄ are bothmethyl groups, R₅ is hydrogen and n is 2.

The material of component (a) can be prepared by reacting an organicpolyisocyanate, particularly diisocyanate, with an amine containing aprimary or secondary amine group and a tertiary amine group for theaminourea embodiment or with an alcohol or polyol containing a tertiaryamine for the aminourethane embodiment. Suitable polyisocyanates includealiphatic, cycloaliphatic, or aromatic polyisocyanates. Diisocyanatesare particularly suitable, although higher polyisocyanates can be used.Examples of suitable aromatic diisocyanates are 4,4′-diphenylmethanediisocyanate, 1,3-bis(1-isocyanato-1-methylethyl)benzene and derivativesthereof, and toluene diisocyanate. Examples of suitable aliphaticdiisocyanates are straight chain aliphatic diisocyanates such as1,6-hexamethylene diisocyanate and cycloaliphatic diisocyanatesincluding isophorone diisocyanate and 4,4′-methylene-bis-(cyclohexylisocyanate). Examples of suitable higher polyisocyanates are1,2,4-benzene triisocyanate, polymethylene polyphenyl isocyanate and theisocyanurate of isophorone diisocyanate. Isophorone diisocyanate isespecially suitable.

Examples of amines containing a primary or secondary amine group and atertiary amine group are dimethylaminopropylamine,bis(dimethylamino)propylamine and 2-amino-5-diethylaminopentane. Anexample of an alcohol containing a tertiary amine isdimethylaminopropanol. Dimethylaminopropylamine is particularlysuitable.

The diamine or amino alcohol and polyisocyanate are combined in anequivalent ratio of about 1:1. The diamine is heated to about 50° C.,and the polyisocyanate is added over a period of time in the range ofabout one to two hours, usually about two hours. The amino alcoholtypically should be heated to about 80° C. before the polyisocyanate isadded. The temperature of the reaction mixture generally increases andis held at an elevated temperature, such as 130° C. to 170° C., untilthe polyisocyanate is completely reacted.

The present invention is further directed to a curable powdercomposition comprising a polyepoxide and the reaction product of apolyisocyanate and either an amine comprising a primary or secondaryamine group and a tertiary amine, or an alcohol or polyol containing atertiary amine. Suitable amines and alcohols/polyols, and the method forpreparing such a reaction product, are described above.

In one embodiment, the material of component (a) further comprises anacidic hydrogen-containing compound; for example, component (a) cancomprise the reaction product of (i) a compound having Formula I and(ii) an acidic hydrogen-containing compound. The acidichydrogen-containing compound of (ii) may be a carboxylic acid, aphenolic compound, a polyester, a polyurethane or an acrylic polymer.Phenolic compounds, especially polyphenols, are particularly suitable.Examples of suitable acidic hydrogen-containing compounds includebenzoic acid, dodecanedioic acid, azelaic acid, itaconic acid, sebacicacid, and adipic acid. Suitable phenols include phenol itself andpolyphenols such as resorcinol, catechol, hydroquinone,bis(4-hydroxyphenyl)-2,2-propane (Bisphenol A),bis(4-hydroxyphenyl)-1,1-isobutane, bis(4-hydroxyphenyl)-1,1-ethane,bis(2-hydroxyphenyl)-methane, 4,4-dihydroxybenzophenone, and1,5-dihydroxynaphthalene. Bisphenol A is especially suitable.

The reaction product used in the coatings of the present invention canbe prepared by mixing the material having Formula I of (i) with theacidic hydrogen-containing compound of (ii) in an equivalent ratio ofabout 1:1 to 1:2, such as about 1:1.87. The material of (i) is typicallyheated to a temperature of about 140 to 180° C. and the acidichydrogen-containing compound of (ii) is added. The reaction mixture isthen usually held at the elevated temperature until it turns clear,indicating homogeneity of the reaction mixture. The reaction mixture isthen allowed to cool.

Component (a) in the present compositions, both with and without theacidic hydrogen-containing compound, is used as a catalyst, andtypically has a melting point of between about 23° C. and 150° C., suchas between about 50 and 100° C. This range of melting points helpsprevent any curing from taking place in the composition before theapplication of heat. This improves the long-term stability of curablecompositions in which component (a) is used. The melting point of thecatalyst is typically not so high, however, that the presentcompositions lose their characterization as “low-cure”. It is thereforedesirable that the catalyst used in the present compositions have amelting point of between about 23° C. and 150° C.; if the melting pointwere too far above this number, the composition might not cure in thedesired manner, and at temperatures too much below this temperature, thecomposition may not be as stable.

The polyepoxides used in the present compositions are those that aresuitable for use in powder coatings, such as those that contain at leasttwo 1,2-epoxide groups per molecule. In general, the epoxy equivalentweight can range from about 180 to about 4000 based on solids of thepolyepoxide, such as between about 500 and 1000. The polyepoxides may besaturated or unsaturated, and may be aliphatic, alicyclic, aromatic, orheterocyclic. They may contain substituents such as halogens, hydroxylgroups, and ether groups.

Suitable classes of polyepoxides include epoxy ethers obtained byreacting an epihalohydrin such as epichlorohydrin with a polyphenol inthe presence of an alkali. Suitable polyphenols include resorcinol,catechol, hydroquinone, bis(4-hydroxyphenyl)-2,2-propane (Bisphenol A),bis(4-hydroxyphenyl)-1,1-isobutane, bis(4-hydroxyphenyl)-1,1-ethane,bis(2-hydroxyphenyl)-methane, 4,4-dihydroxybenzophenone, and1,5-dihydroxynaphthalene. The diglycidyl ether of Bisphenol A isespecially suitable.

Other suitable polyepoxides include polyglycidyl ethers of polyhydricalcohols. These compounds may be derived from polyhydric alcohols suchas ethylene glycol, propylene glycol, butylene glycol, 1,6-hexyleneglycol, neopentyl glycol, diethylene glycol, glycerol, trimethylolpropane, and pentaerythritol. These compounds may also be derived frompolymeric polyols such as polypropylene glycol.

Examples of other suitable polyepoxides include polyglycidyl esters ofpolycarboxylic acids. These compounds may be formed by reactingepichlorohydrin or another epoxy material with an aliphatic or aromaticpolycarboxylic acid such as succinic acid, adipic acid, azelaic acid,sebacic acid, maleic acid, 2,6-naphthalene dicarboxylic acid, fumaricacid, phthalic acid, tetrahydrophthalic acid, hexahydrophthalic acid, ortrimellitic acid. Dimerized unsaturated fatty acids containing about 36carbon atoms (Dimer Acid) and polymeric polycarboxylic acids such ascarboxyl terminated acrylonitrile-butadiene rubber may also be used inthe formation of these polyglycidyl esters of polycarboxylic acids.

Polyepoxides derived from the epoxidation of an olefinically unsaturatedalicyclic compound are also suitable for use in the curable compositionof the present invention. These polyepoxides are nonphenolic and areobtained by epoxidation of alicyclic olefins with, for example, oxygen,perbenzoic acid, acid-aldehyde monoperacetate, or peracetic acid. Suchpolyepoxides include the epoxy alicyclic ethers and esters well known inthe art.

Other suitable polyepoxides include epoxy novolac resins. These resinsare obtained by reacting an epihalohydrin with the condensation productof aldehyde and monohydric or polyhydric phenols. A typical example isthe reaction product of epichlorohydrin with a phenol-formaldehydecondensate.

The curable composition of the present invention may contain onepolyepoxide or mixtures of polyepoxides.

Typically, the polyepoxide is present in the curable composition of thepresent invention in a range of from about 20 to about 90 percent, suchas about 30 to 60 percent, based upon total weight of the curablecomposition. The catalyst or reaction product is typically present inthe compositions of the invention in a range of from about 0.5 to 10weight percent, such as 3 to 5 weight percent. It is expected that therate of cure increases as the concentration of catalyst increases, andthat these increases are directly proportional. It is surprising,however, that no decrease in chemical or mechanical stability is notedas higher catalyst levels are used; stability often behaves inverselyproportional to reactivity, in that as reactivity increases, stabilitydecreases. This maintained stability with increased reactivity is yetanother advantage of the present invention.

The powder coating compositions of the present invention may optionallycontain additives such as waxes for flow and wetting, flow controlagents, such as poly(2-ethylhexyl)acrylate, degassing additives such asbenzoin and MicroWax C, adjuvant resin to modify and optimize coatingproperties, antioxidants and the like. These optional additives, whenused, can be present in amounts up to 10 weight percent, based on totalweight of the coating composition, and if used will typically compriseabout 1 to 5 weight percent. Any of various pigments standardly used inthe powder coatings art can also be included. Pigment weight can be upto 80 percent of the weight of the entire coating and usually is around35 weight percent of the coating. The compositions can further comprisea plurality of particles, such as organic or inorganic particles, ormixtures thereof, that contribute to the mar and/or scratch resistanceof the coatings. Such particles are described in Ser. No. 10/007,149,filed on Dec. 5, 2001, which is hereby incorporated by reference.Pigments or solid additives in nanoparticulate form can also be includedin the present compositions for the same purpose.

It is both a significant and surprising discovery that the presentcompositions will cure at low temperatures in the absence of anyadditional components, such as a crosslinking agent and/or acceleratortypically used in conjunction with polyepoxide resins, and thought to berequired. In some cases, the use of a crosslinker and accelerator canactually raise the temperature required to cure the polyepoxide, sotheir use may be undesirable for a low-cure product. Although theinventors do not wish to be bound by any mechanism, it is believed thatthe reaction product or catalyst used in the present compositioncatalyzes the reaction of the polyepoxide molecules with themselves.This is in contrast to the standard mechanism of action, in which such acatalyst would be expected to facilitate the reaction between thepolyepoxide and crosslinking agent. Thus, the present invention isfurther directed to a method for initiating self cure of a polyepoxideby adding any of the catalysts described herein to a compositioncomprising a polyepoxide.

Notwithstanding the lack of a crosslinking agent, the crosslinkeddensity of the present coating compositions can still be controlled to alarge extent. This is accomplished by controlling the amount of catalystadded to the composition. Higher amounts of catalyst usually gel thefilms faster and crosslink the films more efficiently. In addition,there is a cost savings associated with the elimination of crosslinkersand accelerators, and the ability to cure at a lower temperature.Significantly, the present crosslinker-free and accelerator-freecompositions result, upon curing, in coating compositions that haveperformance properties at least equal to that of conventional powdercoatings in which a polyepoxide and conventional crosslinker are used.This refers to the ability to maintain appearance as measured by anumber of properties relevant to cured coatings, such as resistance tosolvents, pencil hardness, and impact and corrosion resistance.

The present compositions can be prepared by standard methods known inthe art. For example, the components are first thoroughly mixed toensure spatial homogeneity of the ingredients. The composition is thenintimately melt kneaded in an extruder. Typical zone temperatures duringextrusion range from 40° C. to 125° C., such as 45° C. to 100° C. Theexiting extrudate is rapidly cooled to terminate polymerization. Theresulting chip is then micronized into powder with an average particlesize of 0.1 to 200 microns, such as 1 to 100 microns. Comminutionmethods are well known, comminution can be accomplished, for example, byair-classifying mills, impact mills, ball mills, or otherfracture-induced mechanisms. Post additives that improve fluidization ofthe powder mass and/or improve the resistance to impact fusion may beincorporated into the final product before or after micronization. Asnoted, the use of standard powder coating preparation methods is anotheradvantage of the present invention.

Accordingly, the present invention is further directed to powder coatingcompositions that cure at a temperature of between 80° C. and 125° C.comprising a resin and curing agent and wherein substantially all of thecuring agent is extruded with the resin. “Substantially all” means theamount of curing agent needed to completely cure the resin. The presentinvention is further directed to such compositions that do not cure attemperatures below about 70° C., such as at ambient temperature, likemany commercially available low-cure products.

Typically, the present powder coatings will have average particle sizesthat range between 15 and 200 microns, such as between about 25 and 50microns.

The powder coating compositions of the present invention can be appliedto a substrate in any number of ways, most often by electrostaticspraying. The powder coating can be applied in a single sweep or inseveral passes to provide a film having a thickness after cure of fromabout 1 to 10 mils (25 to 250 microns), usually about 2 to 4 mils (50 to100 microns). Other standard methods for coating application can also beemployed.

After application, the present compositions may be cured by heating to atemperature of between about 80° C. and 190° C., preferably betweenabout 80° C. and 125° C., for a period ranging from about 3 minutes to30 minutes, such as 15 to 20 minutes. Heating can be effected by anymeans known in the art, typically by placing the coated substrate in anoven. IR radiation can also be used to heat cure the coated substrates.

Accordingly, the present invention is further directed to a method forcoating a substrate comprising applying to the substrate one or more ofthe coating compositions described herein and curing the coating at atemperature of between about 80° C. and 190° C., such as between about80° C. and 125° C. or between about 105° C. and 120° C. In such amethod, the polyepoxide will self-cure, or react with itself byhomopolymerization; this reaction is catalyzed by the present tertiaryaminourea or tertiary aminourethane compositions. Accordingly, thepresent invention is further directed to a cured coating layercomprising a polyepoxide and one or more of the catalysts describedherein, wherein the polyepoxide is self-cured.

A number of substrates are suitable for coating according to the methodsof the present invention, including plastics such as thermosets orthermoplasts, cardboard, paper, wood, metal, particleboard and mediumdensity fiberboard or mixtures thereof. Substrates coated according tothe present methods are also within the scope of the present invention.

The present invention is further directed to a catalyst composition thatis the reaction product of:

(i) a material having the structure of Formula II and

(ii) an acidic hydrogen-containing compound

For Formula II, R₁ is an organic radical having 6 to 25 carbon atoms, R₂is an organic radical having 1 to 20 carbon atoms; R₃ and R₄ areindependently alkyl or phenyl groups having 1 to 8 carbon atoms; Z isoxygen or nitrogen and when Z is oxygen R₅ is absent and when Z isnitrogen R₅ is hydrogen or

and n is 1 to 4, but when Z is nitrogen, R₂ is an alkylene havingbetween 1 and 4 carbon atoms, and R₃ and R₄ are both alkyl groups havingbetween 1 and 4 carbons, R₅ is not hydrogen.

The present invention is directed to yet another catalyst compositioncomprising the compound of Formula I as described above, wherein thecomposition does not include an acidic hydrogen-containing compound. Ithas surprisingly discovered that compounds having Formula I function aslow-cure catalysts even in the absence of acidic hydrogen-containingcompounds.

As used herein, unless otherwise expressly specified, all numbers suchas those expressing values, ranges, amounts or percentages may be readas if prefaced by the word “about”, even if the term does not expresslyappear. Also, any numerical range recited herein is intended to includeall sub-ranges subsumed therein. As used herein, the term “polymer”refers to oligomers and both homopolymers and copolymers; the prefix“poly” refers to two or more.

EXAMPLES

The following examples are intended to illustrate the invention, andshould not be construed as limiting the invention in any way.

Example 1

The following ingredients were used to prepare a catalyst of Formula I,wherein an acidic hydrogen-containing compound is used.

Percent by Ingredient Weight, g Equivalents weightDimethylaminopropylamine 204.4 1.000 23.95% Isophorone diisocyanate(“IPDI”)¹ 222.2 1.000 26.05% Bisphenol A (“BPA”)² 426.6 3.74 50.00%¹Available from Hüls America, Inc. ²4,4′-Isopropylidenediphenol,available from Dow Chemical Co.

The dimethylaminopropylamine was charged to a suitable reactor andheated to 50° C. The IPDI was added through an addition funnel over aperiod of two hours. The temperature of the reaction mixture was allowedto increase to 90° C. during the addition. After the addition wascomplete the reaction mixture was heated to 130° C. and held at thattemperature until infrared analysis indicated consumption of theisocyanate. The reaction mixture was then heated to 160° C. and theBisphenol A was added. The reaction mixture was held at 160° C. untilthe solution turned clear, indicating complete melting of the BisphenolA. The reaction mixture was poured out hot and allowed to cool andsolidify. The final solid product had a solids content of about 98percent and a number average molecular weight of 336 as measured by gelpermeation chromatography using polystyrene as a standard.

Example 2

The following ingredients were used to prepare a catalyst of Formula I,wherein an acidic hydrogen-containing compound is not used.

Percent by Ingredient Weight, g Equivalents weightDimethylaminopropylamine 204.4 1.000 47.9% Isophorone diisocyanate(IPDI) 222.2 1.000 52.1%

The dimethylaminopropylamine was charged to a suitable reactor andheated to 50° C. The IPDI was added through an addition funnel over aperiod of two hours. The temperature of the reaction mixture was allowedto increase to 90° C. during the addition. After the addition wascomplete the reaction mixture was heated to 130° C. and held at thattemperature until infrared analysis indicated consumption of theisocyanate. The reaction mixture was poured out hot and allowed to cooland solidify. The final solid product had a solids content of about 98percent and a number average molecular weight of 336 as measured by gelpermeation chromatography using polystyrene as a standard.

Example 3

Samples 1 to 4 were prepared using the components and amounts shown inTABLE 1, including the products prepared according to Examples 1 and 2.The coatings were prepared by premixing the ingredients in a three-blademixer rotating at 3500 rpm. The premix was then extruded in a 19 mm dualscrew extruder operating at a temperature of 80° C. The extrudate wasrapidly cooled and pressed into chip. The chip was micronized to anaverage particle size of 35 microns using a Hosokawa Air-ClassifyingMill (ACM).

TABLE 1 Sample 1 Sample 2 Sample 3 Sample 4 EPON 1001³ 340 g 340 g EPON2002⁴ 140 g 140 g DER 642⁵ 480 g PD 9060 (GMA 480 g Acrylic)⁶ Product ofExample 1 15 g 15 g 15 g Product of Example 2 7.5 g Benzoin⁷ 4 g 4 g 4 g4 g Modaflow⁸ 9 g 9 g 9 g 9 g Goresil 210⁹ 50 g 50 g 50 g 50 g TiO₂ 150g 150 g 150 g 150 g ³EPON 1001 is a BPA epoxy, standard hybrid type,with an EW = 550 from Resolution Performance Products. ⁴EPON 2002 is aBPA epoxy, standard hybrid type, with an EW = 750 from ResolutionPerformance Products. ⁵DER 642 is a NOVALAC resin from Dow Chemical. ⁶PD9060 is a GMA Acrylic resin from Anderson Development. ⁷Added as adegasser. ⁸An acrylic copolymer flow additive, anti-crater additive,from Solutia, Inc. ⁹Silica particles, average particle size 2 microns,largest particle size 10 microns, from CED Process Minerals, Inc.

The coatings were sprayed onto Bonderite 1000 steel panels and cured at115.6° C. for 25 minutes. Following cure, the panels were subjected to anumber of tests standard in the industry for testing coatings. Tests andresults are shown in TABLE 2.

TABLE 2 Sample 1 Sample 2 Sample 3 Sample 4 100 MEK double No scuff Noscuff No scuff No scuff rubs¹⁰ Impact Reverse/ <20/<20 70/100 160/160160/160 Direct¹¹ QUV 340 400 hrs¹² 60 → 60 60 → 20 60 → 15 60 → 15Appearance¹³ PCI = 1 PCI = 7 PCI = 7 PCI = 6 Gel time¹⁴ 6:00 3:00 3:003:00 1000 hrs salt fog  2 mm <1 mm <1 mm <1 mm 100 F.¹⁵ creep creepcreep creep 1000 hrs cond hum <1 mm <1 mm <1 mm <1 mm 100 F.¹⁶ creepcreep creep creep Powder stability 6:00 3:00 3:00 3:00 (chemical)¹⁷¹⁰Powder Coatings Institute (“PCI”) #8 Recommended Procedure. (“Noscuff” means the coating is fully cured.) ¹¹ASTM D2794 (Range <20 to 160in*lbs.; 160 in*lbs = full flexibility.) ¹²ASTM D4587 (results reportedin 20° gloss readings taken initially → after 400 hours of QUVexposure.) ¹³PCI visual standards (Range 1 to 10 - 10 being thesmoothest.) ¹⁴PCI #6 Recommended Procedure (gel time reported inminutes:seconds.) ¹⁵ASTM B117 (<1 mm = no salt fog effect.) ¹⁶ASTM D1735(<1 mm creep = no humidity effect.) ¹⁷PCI #1 Recommended Procedure at32° C. (stability reported in minutes:seconds.)

The results in TABLE 2 confirm that a variety of polyepoxy resins can becured at low temperature according to the present invention. The acrylicsample (Sample 1) performed as would be expected in the impact and QUVtesting—that is, not as well on the former and very well on the latter.Bisphenol A epoxies (Samples 3 and 4) work especially well with thepresent invention providing the highest level of impact resistance,humidity and salt fog resistance, and chemical resistance; although QUVresults were lower than with other samples, this would be expected withthis type of resin. One skilled in the art could choose the appropriateresin based on the desired qualities of the cured coating, using thepresent catalysts to effect cure at low temperatures.

Example 4

Sample 3 prepared as described above was tested for stability usingstandard techniques as discussed below. The stability of Sample 3 wasalso compared with the stability of Sample 5, prepared in the samemanner as Sample 3 except using three grams of 2-methyl imidazole as thecatalyst instead of the catalyst prepared according to Example 1. Astandard polyepoxide resin cured with an acid polyester was alsocompared (PCF 80147, commercially available from PPG Industries, Inc.).The coatings were applied as described in Example 2. However, thecommercially available product was cured at a higher temperature (162.8°C.) compared to 115.6° C. for Sample 3 and Sample 5.

TABLE 3 PCF 80147 Sample 3 Sample 5 Mechanical Stability One week at 32°C. Excellent Excellent Excellent Chemical stability Initial Gel @ 145°C. 4:00 3:00 1:30 Gel after One Week @ 4:00 3:00  :40 32° C. 100 MEKDouble rubs No Scuff No Scuff No Scuff

The chemical stability and mechanical stability tests were identical,and were performed by placing virgin, free-flowing powder in a sealedjar and setting the jar in a water bath heated (PCI #1 RecommendedProcedure, as described in TABLE 2). After one week the samples wereevaluated for mechanical stability using a visual ranking. Afree-flowing powder is excellent; the ranking standardly used in theindustry is as follows: excellent>good>cakey>clumpy>fused>sintered. Allsamples had an excellent mechanical stability.

After the visual ranking for mechanical stability, gel times of the agedpowder were taken as per PCI #6 Recommended Procedure to assess thechemical stability of the powder coating. A slower gel time translatesto advancement in molecular weight. A powder coating should not havemolecular weight advancement during storage. As shown in TABLE 3, onlySample 5 (2-methyl imidazole catalyst) showed advancement; thecommercial product and the product of the present invention did notadvance over time.

Solvent cure (100 MEK double rubs—PCI #8 Recommended Procedure) was usedas an indication of film cure. When a film has excellent solventresistance, that is a good indication that complete cure has occurred.Sample 3 of the present invention underwent complete cure just as theother samples tested.

Thus, the low-cure composition of the present invention performed equalto a commercially available high cure product using conventionalcrosslinkers and performed better than a sample using a low temperaturecuring agent outside the scope of the present invention.

Whereas particular embodiments of this invention have been describedabove for purposes of illustration, it will be evident to those skilledin the art the numerous variations of the details of the presentinvention may be made without departing from the invention as defined inthe appended claims.

1. A curable powder composition comprising: (a) a polyepoxide; and (b)the reaction product of a polyisocyanate and an amine comprising aprimary or secondary amine group and a tertiary amine.
 2. Thecomposition of claim 1 wherein the polyisocyanate is a diisocyanate. 3.The composition of claim 2, wherein (b) is mixed with an acidichydrogen-containing compound.